There are two aspects to the problem faced by current GSM-UTRAN (global system for mobile communications-universal terrestrial radio access network) capable MS (mobile station) roaming in a GSM-UTRAN network environment. One aspect is that the MS may keep trying to reselect the same cell (e.g., UTRAN cell) that was already found to be unsuitable. The other aspect is the MS is wasting power resources by monitoring cells which it has no access to. These two situations are elaborated below.
The first situation is when the MS is trying to reselect the same unsuitable cell (e.g., the UTRAN cell). Currently, a 3GPP (Generation Partnership Project)-standardized inter-RAT (radio access technology) capable MS monitors cells of other radio access technologies for MS-based cell reselection evaluation when they are provided in a network-defined neighbor list. When the radio based criteria for reselecting another radio access technology cell are met, the MS may attempt to reselect a particular best cell if it is suitable. Details of the algorithm for cell reselection (using defining a ranking quantity, e.g., having the greatest received signal code power (RSCP), to specify the best cell), according to the prior art, e.g., from GSM to UTRAN, are provided in “3rd Generation Partnership Project; Technical Specification Group GSM/EDGE Radio Access Network; Radio subsystem link control”, 3GPP TS 45.008, section 6.6.5.
If the conditions for successful camping onto the cell are not met (e.g., not a suitable cell or the access on the cell is forbidden), the MS may return to the originating radio access technology (e.g., GSM), where it starts measuring and evaluating other radio access technology cells that are provided in a network-defined neighbor list again. The same cell that was earlier tried may still be evaluated as the best cell such that the MS tries to reselect to this cell again only to find out again that it is not able to camp onto it. The MS returns to the originating radio access technology and the cycle perpetuates. FIG. 1 illustrates the situation when a mobile station is trying to reselect the same unsuitable cell, per prior art. Thus, the prior art does not provide any guidance on how the MS could handle this situation.
The second situation is when the MS is wasting power resources by monitoring forbidden access cells (e.g., the UTRAN cell). In particular, to find out if an access is allowed on a cell with no history of the forbidden access cells in 3GPP cellular systems, the MS has to perform a location update procedure in order to register to the network. When the MS gets a Location Update Reject message from the network with reject cause values #13 or #15, this means that MS access on this cell is forbidden. According to “3rd Generation Partnership Project; Technical Specification Group Core Network; Mobile radio interface Layer 3 specification; Core network protocols; Stage 3”, 3GPP TS 24.008, 4.4.4.7, the MS shall then store the Location Area Identity (LAI) on a list of “forbidden location areas for roaming”. This information is used in the future cell selection attempts where the MS is able to check if a target cell is not belonging to a forbidden LAI. This check is one of the criteria that needs to be met for a suitable cell. However, the LAI information is not sufficient to uniquely identify a cell. More than one cell in an area may have the same LAI. Thus, it is possible that the MS is continuously measuring the same cell or cells belonging to the same forbidden LAI when the MS only stores the LAI.
In certain network environment, in particular shared cellular networks, due to the changed business conditions, it is increasingly common to find an operator operating only a WCDMA network and having a roaming agreement with another operator owning another radio access technology network to extend the coverage, e.g., Hutchinson 3 subscribers roam in Austria's Mobilkom GSM network. The latter operator may also operate its own WCDMA network which is not accessible to the roaming partner's subscribers and vice versa. The network-defined neighbor list may only contain cells which the partner's subscribers have no access to. The reason why such an MS ends up receiving such a list is because this information is broadcasted to all MS camped on the same cell.
It is possible for the network to use existing signaling mechanisms to customize a measurement trigger threshold and neighbor list for a particular MS such that it will not monitor such cells under certain conditions, which requires the network to have very tight control over the MS mobility which may not be practical from the implementation point of view in terms of the complexity and signaling load. From the network operator implementation point of view, the easiest way would be for all MS camped onto this cell to use the broadcast-defined neighbor list to measure the cells. Since it is possible that the subscribers camped on the cell may be the owner operator's subscribers and/or its roaming partner's subscribers due to the shared network, the measurement threshold can be also set such that the MS will measure the cells, if the network does not want to specially customize the settings for a particular MS. The end result is the roaming partner's subscriber terminals end up measuring and evaluating cells which they have no access to, i.e., cells belonging to a forbidden LAI.
For the MS to obtain information on whether the target UTRAN cell, i.e., the cell to be reselected to, is suitable or to find out if the access is allowed, means that there is a period of time where the MS is not able to provide service to the end user in terms of missing incoming call pagings, inability to process mobile-originated call requests and disruptions in data transfers.
Even though the MS had already “discovered” that all cells of a particular LAI are forbidden, it still needs to read the system information messages of the target cell to obtain the LAI. Thus, it is not possible to avoid the gap in service availability, unless the implementation uses a dual GSM & WCDMA radio frequency (RF) receiver hardware—one RF for each RAT—but this is not cost effective and increases the number of components for manufacturing. A number of these components increases even more if there is another cellular RAT, e.g., CDMA2000, incorporated in a phone product.
At least in the context of a GSM to WCDMA cell reselection and 2G-2G cell reselection, currently the MS monitors such cells because they have been provided in the network-defined neighbor list. It may not end up reselecting such cells after the first location update attempt on the cell due to the suitability criteria not being met, but it continues to measure and evaluate these cells. The successful evaluation of these cells based on the radio criteria would trigger the MS to check the suitability of the cell. To check this, the MS needs to receive the system information messages of the target cell which would cause a short gap in service availability for a shared GSM/WCDMA radio frequency receiver hardware implementation.
FIG. 2 shows a flow chart illustrating how a mobile station is wasting resources in monitoring forbidden access cells, per prior art, as described above.